This invention relates to a fuel injection system for a diesel internal combustion engine, and more particularly, to a diesel engine fuel injection system that utilizes accumulator nozzles, to inject diesel fuel into the engine cylinders and that employs a multiple pumping element or chamber jerk pump for providing the system with pressurized diesel fuel.
Jerk pump fuel injection systems for diesel engines are known. In such systems, each engine cylinder has its own pumping element. An inlet or fill port allows the inflow of fuel to the pumping chamber of the pump. Before ingress into the pumping chamber, the fuel is stored in a fuel gallery and is pre-pressurized, to about 50 psi, by a separate charging pump.
When the pressure of the fuel in the pumping chamber of the jerk pump is high enough, the pump's delivery valve opens and fuel passes into an injection line. The high pressure fuel from the pumping chamber presses against the fuel in the injection line causing a compressive wave to propagate down the injection line at a velocity of sound in fuel (approximately 5,000 feet per second).
A nozzle or ejector unit is located at the other end of the injection line, and in essence, closes that end of the line. The initial part of the compressive wave may be reflected at the nozzle back down the injection line toward the pump. The compressive wave pressure quickly builds up to a pressure required to open the nozzle. The open nozzle injects fuel into the engine combustion chamber through tiny orifices. Much of the fuel driven toward the nozzle by the compressive wave is injected into the engine combustion chamber in this manner. The match among the injection line internal diameter, the total nozzle orifice area and the pressure of the compressive wave determines the fraction of fuel that is reflected during the injection.
Referring back to the jerk pump, the pumping piston completes its mission by opening its spill port. As a result, fuel is dumped from the pumping chamber back into the fuel gallery. As the pressure in the pumping chamber drops, the delivery valve closes to prevent a back flow from the injection line. The compressive wave is no longer formed, and this lack of pressure is propagated to the nozzle, also at the speed of sound. When the pressure drops, the nozzle closes, ending the injection of fuel into the engine cylinder.
The function of the delivery valve in such fuel injection systems is important and complex. If the delivery valve were a "pure" check valve, those parts of the compressive wave that were reflected at the nozzle would be reflected again at the delivery valve. If this second reflection was strong enough, it could open the nozzle again when it arrives there. This "secondary" injection would occur too late to contribute to engine power but in time to partially burn. Such a late burn wastes fuel and forms excessive smoke even by standards existent before the recent emphasis on pollution control.
To prevent such a secondary injection, delivery valves generally include structure that permits a fixed amount of fuel to be by-passed back into the pumping chamber before the delivery valve is allowed to seal. As a result, the residual pressure in the injection line is brought close to zero between injections. It has been recognized that there are perhaps more disadvantages to the use of a delivery valve than advantages; yet their usage continues.
The time of the injection of fuel into the engine cylinders controls the timing of the heat release as the fuel burns. Before control of emissions became so important, the fuel was injected generally twenty degrees before the engine piston reached its top dead center. Pressure and temperature in the engine chamber is rapidly rising at this time, and fuel would have its combustion delayed until just before top dead center. As the cycle preceded past top dead center, the heat release was very fast, creating high pressures and temperatures at the peak of the stroke. Oxides of nitrogen ("NOx") were formed rapidly under these conditions, and even though the fuel had a relatively long time to burn, there was also significant smoke and particulates produced.
It is known that substantial reductions in NOx pollutants can be achieved, without much penalty in fuel consumption, if the timing is retarded to approximately five degrees before top dead center. However, such timing is now "on the edge". In other words, a little more retarding and the fuel consumption goes up quickly; a little less retarding, and the NOx pollution goes up quickly.
Because of the hydraulics of the jerk pump system, there is a natural retarding as the engine speed increases. This is caused by the fixed time that it takes for the pressure wave to travel down the injection line. Hence, to maintain optimal timing at all engine speeds, the current practice is to use timing devices such as, for example, of the type generally described in Berman and DeLuca, FUEL INJECTION AND CONTROLS FOR INTERNAL COMBUSTION ENGINES (Simmons-Boardman Publishing Corporation, 1962) at pages 166-168 ("Berman"). Others have, however, suggested electronically timed injection to optimize conditions for both engine load and speed.
Control of pollutants caused by the operation of diesel engines has and continues to be an important motivating factor in designing fuel injection systems for diesel engines. Pollutants fall into two general classes: one is the NOx's caused by allowing the combustion chamber's temperature to be too high for too long; and the other is in the form of smoke, particulates, carbon monoxide and odor that are formed by incomplete combustion. As noted above, the control of the timing of the fuel injection has been a primary means utilized to control NOx pollutants, with the conventional wisdom being that the later the start of the fuel injection, the lower the NOx pollutants. Unfortunately, later injection timing tends to decrease engine efficiency and to increase the other class of pollutants.
Under most conditions of steady state diesel engine combustion, there is plenty of air to burn the fuel. Incomplete combustion is caused by the fuel not vaporizing in time or the fuel vapor not getting mixed with the air in time. In the short time that combustion takes place, the smallest drops of fuel easily vaporize and complete their combustion. However, relatively larger drops take longer to complete this process, and it is these larger drops, particularly those formed by the low pressure end of the injection, that cause smoke, odor, carbon monoxide and particulate pollutants. In other words, it is only a small part of the injected fuel that leads the "incomplete combustion" class of pollutants.
From the standpoint of fuel injection system design, it has long been recognized that the larger drops of fuel are formed only when the pressure drop across the nozzle orifices is small. In the conventional jerk pump injection system, the latter condition exists as the nozzle needle valve attempts to close the orifices, that is, at the end of the injection cycle.
One proposed way of avoiding ending the injection with a low pressure drop is to raise the closing pressure at the nozzles. This has been done experimentally by applying a hydraulic back pressure to the nozzle needle valve from an external high pressure hydraulic source. This does minimize, to a certain extent, the incomplete combustion class of pollutants, but it is not practical in a working fuel injection system. More specifically, if the closing pressure is raised in a conventional system, the opening pressure is also raised and at cranking speeds, the injection pump cannot raise the pressure of the fuel sufficiently high.
It has been proposed to use accumulator nozzles to inject the fuel in the engine cylinders since such nozzles can be employed to avoid low pressure injection. In an accumulator nozzle, the fuel injection starts at full charging pressure. The nozzle injection pressure drops continuously until it reaches the nozzle closing pressure. The nozzle closes at that pressure such that at no time is fuel injected at a lower pressure. Accordingly, the use of accumulator nozzles with a conventional fuel injection system should theoretically yield less smoke, particulates, carbon monoxide, and odor than do other conventional nozzles. Unfortunately, the accumulator nozzle has opening pressure limitations during engine cranking like those of conventional nozzles.
The use of an accumulator nozzle also has other possible advantages in that it allows a simple method of timing the injection as compared with other conventional injection nozzles. Specifically, the accumulator nozzle does not inject while its accumulator volume is being charged. The injection starts only when the pressure in the injection line is reduced. This can be delayed after the charging is complete without difficulty. For example, if the charging pump completes its charging function and is then isolated from the injection line by, for example, a simple check valve, the injection line pressure can thereafter be reduced by a secondary or spill valve at a time not related to the charging process. If this secondary valve is controlled electronically, all the flexibility of electronic timing can be used to also reduce the other pollutants, that is, the NOx's. Alternatively, the secondary valve may be controlled mechanically by a timing device which is dependent on engine speed and which will advance the injection at higher speeds to obtain an optimum pollutants/fuel consumption relationship.
Because of the evident advantages of accumulator nozzles in diesel engine fuel injection systems, those working in the art have attempted to solve the opening pressure limitations during engine cranking noted above. In this regard and as noted, it has long been recognized that the final injection pressure should be as high as possible. This, of course, can be accomplished by raising the closing pressure of the accumulator nozzle.
Although the conventional jerk pumps have no difficulty in supplying high pressure fuel under normal operating conditions, such pumps, as noted, are often not able to supply high pressure while the engine is cranking. If the closing pressure is held constant, it is limited by the requirement that the fuel injection system be able to start the engine under all conditions. In this regard, it has been recognized that the closing pressure may be very low, under engine starting conditions, and may be raised to a higher value, under engine running conditions, by imposing a back pressure in the chamber whose pressure controls the opening of the nozzle needle valve of the accumulator nozzle. This can be done by regulating the pressure of the sump into which the timing or secondary valve spills. Under this arrangement, the back pressure would be approximately zero while the engine was being cranked because there would not be enough spill flow to raise the pressure. After the engine is running for a short time, the back pressure in the chamber will rise. Thus, the opening pressure (and closing pressure) can be very low for easy starting and the closing pressure can thereafter be raised to a higher value for reduced incomplete combustion pollutants under running condition.
There is, however, one serious problem that has long been recognized with regard to the use of accumulator nozzles in diesel engine fuel injection systems and that, as a practical matter, has prevented the usage of accumulator nozzles (as opposed to more complicated and expensive injector units that include accumulator nozzles as a component part) in such systems. Specifically, the accumulator nozzle that can deliver a certain maximum fuel charge or quantity of fuel has difficulty delivering a small enough charge or quantity to allow the engine to idle satisfactorily. The quantity of fuel discharged may be given by the following relationship: EQU q=V/K(P.sub.1 -P.sub.2)
Where "q" is equal to the discharged quantity in cubic millimeters; "V" is equal to the volume of the accumulator in cubic millimeters; "K" is equal to the bulk modulus of fuel, for example 280,000 psi; "P.sub.1 " is equal to the peak accumulator pressure in psi; and "P.sub.2 " is equal to the nozzle closing pressure in psi.
The ratio of the maximum fuel charge delivered to the minimum fuel charge delivered is called the "turn down ratio." The art has recognized that engines need a turn down ratio of at least 8 and preferably 10.
The injected quantity fuel is stored in the compressibility of the fuel in the accumulator volume. As already stated, the quantity of fuel delivered is proportional to the pressure difference between the starting pressure and the closing pressure. The minimum delivery occurs when the charging pressure P.sub.1 is just enough to open the nozzle, that is, just greater than the opening pressure. The minimum delivery is proportional to the opening and closing pressure difference, e.g., 1,000 to 1,500 psi. To obtain a turn down ratio of 8, the maximum charging pressure, P.sub.2, would have to be 8,000 to 12,000 psi higher than the closing pressure. This is quite demanding of a jerk pump. In addition, raising the closing pressure of the accumulator nozzle aggravates the turn down ratio problem. In accumulator nozzles raising the closing pressure raises the opening pressure even more. The larger difference in these pressures would force extremely high maximum charging pressures at full delivery to obtain a reasonable turn down ratio. Such higher pressures would require extra expense, reduced life of the hydraulic equipment, and wasted power.